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Slots Within an Image Plane Affecting RF Current Return
An excessive number of through-holes within a power and ground plane creates a situation commonly identified as the Swiss cheese effect. The physical copper between through-hole pins has been removed leaving large areas of discontinuity. This occurs because during the manufacturing process, oversized drill bits are used. Oversized holes reduce the cost of manufacturing, by allowing a greater tolerance for the manufacturer because they use a larger drill bit (smaller bits break easier).
RF return current flows in the reference plane adjacent to the signal route. Due to this discontinuity, the RF return current cannot mirror image itself to the signal trace on the adjacent layer. As seen in the figure, RF return current in the reference plane must travel around the through-hole slots. This extra RF return path length creates a loop antenna, developing a magnetic field between the signal trace and return path. Additional inductance in the return path also means less magnetic flux cancellation. With through-hole components that have copper between pins (non-oversized holes), reduction in magnetic flux propagating from the transmission line is achieved.
If the signal trace is routed "around" through-hole discontinuities (the left-hand side of the figure), a constant RF return path cannot be maintained along the entire signal route. The opposite condition is true for the right-hand side of the slot. No continuous image plane is present, hence there is not a short, direct-line RF return path. Problems can however arise when a signal trace travels through the middle of slotted holes. When routing traces between through-hole, provisions must be made for at least three times the width of the trace between the signal trace and though-hole location to minimize crosstalk opportunities between trace and pin.
Equivalent circuit showing inductance in the return path. This inductance is approximately 1 nH/cm.
Generally, slots in a PCB with oversized or overlapping holes will not cause RF problems for the majority of signal traces routed between through-hole device leads. This observation is valid for traces that do not carry high levels of RF energy or static level signals-not clocks or period signal traces. For high-speed, high-threat signals, alternative methods of routing traces between through-hole component vias must be devised.
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